Project Summary: The goal of this project is to establish a murine Group A Streptococcus (GAS) transmission model that leads to acute infection in the mucosa of the upper respiratory tract in recipient mice. GAS is a major human pathogen that causes common pharyngeal and skin infections. GAS also occasionally causes severe invasive infections such as pneumonia, necrotizing fasciitis, bacteremia, and toxic shock syndrome. Murine and nonhuman primate infection models have been valuable for understanding GAS pathogenesis; however, these models involve high GAS inocula and cannot be used to investigate the onset of pharyngeal GAS infections. Consequently, there is a significant knowledge gap regarding immune protection and GAS pathogenesis during the onset of acute pharyngeal GAS infection. Another knowledge gap is the lack of understanding of the basis for the differential susceptibility to infectious respiratory diseases between males and females. We recently found that wild-type M1T1 GAS is effectively cleared from the lung in a murine intratracheal pneumonia infection model but not from the skin with subcutaneous infection and that the clearance of GAS from the lung requires the gp91Phox (Nox2) gene, which encodes the ? chain of the catalytic subunit of the NADPH oxidase. These findings inspired us to test whether gp91phox knockout (KO) nave mice develop acute GAS infection in the nasopharynx and oropharynx after they are comingled with donor mice that were inoculated in the nostrils with GAS. Pilot tests showed that male but not female gp91phox KO recipient mice acquired acute infection in the mucosal epithelium of the upper respiratory tract within 6 days after comingling. Based on these exciting and surprising preliminary results, we propose to establish a Group A Streptococcus transmission model for male- biased acute infection in the mucosal epithelium of the upper respiratory tract in gp91phox KO recipient mice (Aim 1) and determine the basis for the gender bias in acute Group A Streptococcus infection in the mucosal epithelium of the upper respiratory tract (Aim 2). The successful execution of this project would provide a novel GAS transmission model that will be invaluable for investigation of innate immune protection and GAS pathogenesis during the onset of pharyngeal GAS infections. This project also has the potential to provide a new paradigm to explain the gender differences in susceptibility to respiratory infections.